Mixers are used in transceivers, such as mobile phones and Wi-Fi devices, to convert low frequency signals to high frequency signals, and high frequency signals to low frequency. Mixers essentially receive two signals at different frequencies and produce several outputs at different frequencies resulting from the mix. Filters are then used to select the output signal having the frequencies of interest.
In general, a mixer makes use of a locally generated artificial RF signal (from a local oscillator), to convert the received signal into the outputs.
In communications receivers and transmitters, a ‘superheterodyne’ mixer indicates the use of more than one mixing stage to step between the radio frequency (RF) signal that is transmitted and the baseband information signal. Superheterodyne architectures generally require off-chip components and are more expensive to implement than single chip solutions.
In contrast ‘homodyne’ refers to the direct conversion between RF and baseband (usually there are at least two quadrature baseband signals). Direct conversion avoids the use of intermediate stages and frequencies, and requires less filters and amplifiers. As a result homodyne circuits are generally more cost-effective to integrate.
There are however drawbacks to direct conversion. For instance, in a receiver where the local oscillators provide an artificial RF frequency at the same frequency as the received RF, then an unwanted DC offset can appear in the baseband output. To address this problem the local oscillators can be arranged to generate a signal at a given fraction of the RF frequency; as a result the artificial RF and received RF are different. Mixers that use this technique are termed ‘sub-harmonic mixers’.
Another problem arises where there is noise in the output of the power amplifier. Where this noise is near to the frequency of the local oscillator it can ‘pull’ the local oscillator frequency to ‘lock’ onto the noise frequency. This corrupts the output but can also be alleviated by sub-harmonic mixing.
Referring now to FIG. 1(a), in a homodyne receiver 10 an antenna 12 receives a modulated RF signal. After reception this signal is passed through a filter 14 and an amplifier 16 before mixing with a locally generated signal at half the RF frequency at mixer 18. The output of mixer 18 passes through another filter and amplifier 20 to provide the analogue baseband signal. A digital baseband signal becomes available after processing in the analogue to digital converter (ADC) 22.
Referring next to FIG. 1(b), a transmitter 40 takes a digital baseband signal and passes it through a digital to analogue converter (DAC) 42 to generate an analogue baseband signal. The analogue signal is passed through an amplifier 44 and then mixed with a locally generated RF signal at mixer 46 to up-convert it. Further amplification 48 and filtering 50 results in the RF signal, modulated with the analogue baseband signal, being provided to transmitting antenna 52 for transmission.
The architecture of a conventional mixer, as used in FIG. 1, is shown in FIG. 2. Mixer 60 comprises a first pair of transistors 62 and 64 connected with common source and common drain. The in-phase signal from the local oscillator LO1 is applied across the gates 66 and 68 of this pair of transistors. A second pair of transistors 70 and 72 are also connected with common source and common drain. The quadrature signal from the local oscillator LOQ is applied across the gates 74 and 76 of the second pair of transistors. All four sources are connected together 78. The input voltage is applied to the common source of all four transistors 78 via a further transistor 80 connected between the common source 78 and a matching network 90. The matching network is generally designed using L matching or inductive degeneration. The output Vout is taken from between the common drain 82 of the first pair and the common drain 84 of the second pair of transistors. A matching network may also be provided at the gate of further transistor 80.